1. Field of the Invention
The present invention relates to a piston pump which is used for supplying a hydraulic energy in a motor vehicle.
2. Description of the Related Art
Modern piston pumps for use in driving dynamics control systems for motor vehicles suffer from the problem that new functions require from the pump to have an extended useful life and to operate against high system pressure at an increasing rate. In this case, the system pressure can principally prevail both on the suction side and the pressure side of the piston pump. This problem is encountered, for example, in the case of an electrohydraulic parking brake in which it is desired to trigger the parking brake function by a pressure pulse of the pump.
Circumstances and marginal conditions of this type place very high demands on the components of the pump such as the sealing elements employed in particular. Their resistance to wear and slot extrusion is of special interest.
In the case of slot extrusion, a back of a sealing element on the low-pressure side is urged into a slot as a consequence of the prevailing difference in pressure. To avoid this effect, the sealing element is required to exhibit a certain ability of bridging the slot or a certain shearing resistance. If the shearing resistance is not sufficient, the back of the seal will move into the slot, where it is destroyed. This process is still supported by the fact that the accommodating bore for the piston expands at least slightly when exposed to the internal pressure and compresses again abruptly when relieved from load. The quasi elastic breathing process has as an effect that the sealing material disposed in the slot is unable to re-deform in time, in particular at low temperatures. So-called extrusion tabs can develop at the sealing element which will extend with continuous time of application. In the final stage, the seal will fail because no sufficient sealing function is safeguarded due to changed distributions of radial stress in the sealing element. The major influences for the slot extrusion are the slot ‘S’ that must be sealed radially, the system pressure ‘P’, the radial profile width of the sealing element ‘T’, the nominal diameter ‘d’ and the working temperature ‘T’ as well as the elasticity of the sealing element ‘E’. The extrusion behavior can be influenced by the conception of all mentioned variables. However, mainly the physical material properties of the seal are of decisive importance under thermal load.
Thermoplastic elastomers such as EPDM, thermoplasts such as PTFE as well as materials belonging to the group of the thermoplastic urethanes are used for sealing elements in many cases. The physical properties of the materials will change with rising temperature to the disadvantage of extrusion safety. Usually, TPU materials (thermoplastic polyurethanes) can withstand temperatures up to 100° C. EPDM and PTFE will withstand up to roughly 120° C. In these cases the system pressure is limited so that a combination of maximum temperature at maximum pressure should be avoided. Alternatively, the slide slot ‘S’ should be reduced for such cases, what has considerable consequences related to works economy in the series manufacture.
According to the abstract of JP 2000-110737, a piston is furnished with an annular groove into which a ring seal is embedded that is dynamically sealing at a cylinder wall. To constructively enhance the resistance to extrusion, the document discloses to provide an annular slot at the seal's back between a piston and a piston accommodation in a greatly expanded fashion as well as with a wedge-like tapering profile. The intention of this procedure is to provide the sealing element with an expanded space for deformation into which the sealing element may enter and also exit again elastically.
Potential shearing damage at the sealing element is avoided in the prior art measure by tolerating a reduced support of the sealing element within the groove. This can have as a result a reduced sealing effect and also a reduced guiding function. The reason is that an expansion of the sealing element into the wedge-shaped slot causes a reduction of the contact pressure between cylinder and sealing element.